Bump gap to 4.7.5 for sage 6.3

Package-Manager: portage-2.2.10
bump to 4.7.5 for upcoming sage 6.3. minor adjustment to SRC_URI in 4.7.4 to keep repoman happy.

Package-Manager: portage-2.2.10
masking (lib)gap-4.7.5 ready for sage 6.3